Photonic bandgap structures are man-made periodic lattices, in one, two or three dimensions, which are photonic analogs to semiconductors in electronics. Unlike semiconductors, in which a periodic structure (i.e., the crystal lattice) provides a structure which defines allowed and forbidden energy levels for electron propagation, photonic bandgap materials provide a periodic structure which exhibits a frequency gap where the propagation of electromagnetic waves, including the spontaneous emission of photons with frequencies within the gap, is forbidden for all wave vectors. Depending on the unit cell size of the periodic structure, the crystallographic orientation of the periodic structure, the dielectric constants and indices of refraction of the materials and other factors, these structures can be tailored to propagate electromagnetic waves within only certain wavelength ranges. It has been shown that the dimensions of the periodic lattice have to be on the same order of magnitude as the desired bandgap wavelength. For example, to obtain a bandgap for microwaves and longer wavelengths, the size of the unit cell must be in the millimeter range or greater. Also, similar to semiconductors, "defects" can be introduced, generating energy levels in the forbidden bandgap. This allows for the transmission of selected wavelengths, making the photonic bandgap structures very good optical filters. A schematic representation of a photonic bandgap structure having a periodic lattice in three directions is shown in FIG. 1.
Generally, photonic bandgap structures may be employed in all applications where resonant cavities are utilized, such as filters, impedance matching structures, lasers, etc. Some commercial applications include collision avoidance radar, planar antennas, microwave communications, telecommunication networks, low threshold optical switches, amplifiers and microlasers, and medical imaging.
A conventional photonic bandgap structure may include a mass of a high dielectric matrix material, such as a ceramic, with a lattice of holes extending through the structure in one, two or three dimensions. The holes may be empty (i.e., filled with air or vacuum) or filled with one or more low dielectric materials. Preferably, there should be a difference of at least 2 between the dielectric constant of the matrix material and the dielectric constant of the hole materials. Inverse structures are also possible, with the matrix phase being formed from a material having a relatively low dielectric constant and the hole phase being formed from a material having a relatively high dielectric constant.
In one prior art method for making such structures, hundreds of thousands of tiny holes are drilled in a periodic array through a mass of ceramic in one or more directions. This process is tedious, presents difficulties in obtaining accurate hole size and hole alignment, and does not permit the creation of blind holes within the ceramic mass.
Another prior art process involves the acid-etching of a multiplicity of holes in a single crystal of a ceramic or semiconducting material. In this process, the single crystal is oriented in a preferred direction and a photo-resist material is applied to a surface thereof. The photo-resist material is then selectively cured in a predetermined pattern such that the uncured regions define the pattern of holes which are to extend in one direction through the crystal. The uncured photo-resist material is removed to expose the hole pattern, and the crystal is immersed in an acid or a reactive plasma which attacks the holes, but not the remainder of the surface. After the holes have been formed in the first direction, the process may be repeated to form holes in second and third directions, as desired. In this process, the acid or reactive plasma must attack the crystal material more readily in the direction of the hole than in directions transverse to the direction of the hole so as to prevent ballooning effects and produce holes which have a uniform diameter along their entire extent. Further, it is not possible to produce blind holes using this process.
In yet another prior art process, hundreds of thousands of small spheres of a ceramic material are stacked one at a time in a predetermined array defining the desired latticework of holes. The array is then infiltrated with a polymer which holds the spheres in place and may act as a low dielectric constant matrix phase. This process is obviously tedious, lengthy and costly because of the labor involved.
In still a further prior art process which is similar to chemical vapor deposition processes, reactant gases for forming a desired composition for the hole phase are supplied to a chamber. A plurality of laser beams are moved and focused in the chamber at a point in space at which the combined intensity of the laser beams causes the gases to react and deposit on a selected region of a substrate. In accordance with this process, the laser beams are first moved in a direction away from the substrate to "grow" a thin filament out from the substrate surface. The laser beams are then moved in directions parallel to the substrate surface to "grow" branches out from the filament. By growing multiple parallel filaments and branches interconnecting the filaments, a latticework of a high dielectric constant ceramic can be produced with a desired hole pattern. The latticework can subsequently be infiltrated with a low dielectric polymer to form the matrix phase of the photonic bandgap structure. This process is not only costly, but is limited by the types of materials which can be successfully grown. Also, because the latticework needs to be interconnected in order to support itself, it is difficult to use this process to produce devices having discontinuous lattice structures, including blind holes.